Conventionally, for the attaching device and the attaching method of this kind, as shown in Patent Document 1, it is arranged that a holding section of an ultrasonic sensor module is disposed on the back of a bumper of a vehicle, the ultrasonic sensor module is housed in the holding section from the back of the bumper, and the surface of an assembly section of the ultrasonic sensor module that projects from the end face of the holding section is disposed to be flush with the surface of the bumper.